1. Field of the Invention
The present invention relates to a split-beam type quantitative echo sounder and a method of split-beam type quantitative fish echo sounding.
2. Description of the Related Art
Today, a quantitative echo sounder capable of estimating the length of a single fish and the quantity of a fish school is an essential tool for surveys of fishery resources and efficient fishing operation. Generally, the quantitative echo sounder measures the length of a single fish based on the fact that target strength TS of the fish, which is defined as the ratio of the sound pressure level of an echo signal returned by the fish, or a target of measurement, back to a transducer to the sound pressure level of an incident ultrasonic (acoustic) sounding signal emitted from the transducer, is proportional to the square of the fish length. The transducer however has its own directional characteristics, so that the intensity of a sound wave emitted by the transducer and the receiving sensitivity thereof vary with the direction in which the sound wave is transmitted and from which the fish echo arrives. For this reason, the level of an echo signal received from one fish differs from that received from another fish even if those fishes have the same length. Therefore, if the location of a target fish is offset from the direction of an acoustic axis (or main lobe direction) of the transducer, the level of an echo signal received from the target fish is lower than would be received when the same target fish is located exactly on the acoustic axis. Such a deviation of the target fish from the acoustic axis of the transducer conventionally causes of a measurement error in quantitative fish echo sounding.
One previous approach to the solution of this kind of measurement error problem is found in Japanese Patent Application Publication No. 1994-160522. Specifically, this Publication proposes a method of correcting measurement errors as illustrated in FIGS. 8A–8C. According to the method of the Publication, a transducer is divided into four vibrating elements a–d as shown in FIG. 8A. When radiating a sine-wave acoustic sounding signal, all of the four vibrating elements a–d are excited in phase to together generate sound waves having sharp directivity oriented vertically downward. Upon receiving echo signals, a quantitative echo sounder calculates a phase difference φX between the echo signal received by the vibrating elements a, b and the echo signal received by the vibrating elements c, d and determines the angle of incidence θX that the direction of the incoming echo signal makes with the normal to an X-Z plane taking into account the distance L between a common center of gravity of the vibrating elements a, b and a common center of gravity of the vibrating elements c, d as depicted in FIG. 8B. Similarly, the quantitative echo sounder calculates a phase difference φY between the echo signal received by the vibrating elements a, d and the echo signal received by the vibrating elements b, c and determines the angle of incidence θY that the direction of the incoming echo signal makes with the normal to an Y-Z plane as depicted in FIG. 8C. The quantitative echo sounder corrects the level of the received echo signal by the incidence angles θX, θY to reduce measurement errors of the target strength TS. To eliminate uncertainty involved in determining the incidence angles θX, θY, the aforementioned sine-wave acoustic sounding signal generated by the vibrating elements a–d is frequency-modulated by a sine wave of which frequency is lower than that of the sounding signal itself in this approach.
Another previous approach to the measurement error problem is found in Japanese Patent Application Publication No. 2000-46946. This Publication discloses a quantitative echo sounder employing a transducer including multiple vibrating elements (not shown) which are divided into three groups, that is, a front beam group, a rear-left beam group and a rear-right beam group, the vibrating elements of the three groups together forming a flat radiating surface as illustrated in FIG. 9. Each group includes a large number of vibrating elements to form a receiving beam having sharp directivity. The quantitative echo sounder detects a time difference between echo signals picked up by each pair of receiving beams, converts the time differences detected by individual pairs of receiving beams intersecting at 60 degrees in plan view into time differences which would be obtained if the receiving beams were arranged to intersect at right angles, and determines the angles of incidence of the echo signals from the converted time differences
Although not intended for measuring the level of a received echo signal, another previous approach is found in Japanese Patent Application Publication No. 1982-149908, which discloses a depth sounder to be installed on a survey vessel for measuring water depths with an arrangement for reducing depth measuring errors caused by such motions of the survey vessel as pitching and rolling. By detecting pitch and roll angles of the survey vessel, this depth sounder controls the phases of acoustic signals transmitted and received by individual vibrating elements of a transducer in order to maintain the direction of an acoustic axis of the transducer vertically down.
Still another previous approach is proposed in Japanese Patent Application Publication No. 1993-19053, which discloses a depth sounder provided with a motion sensing device. By using information on pitch and roll angles and acceleration of a vessel output from the motion sensing device and the relationship between the location of a transducer and the location of the motion sensing device, the depth sounder determines displacement Δh of the transducer in the vertical direction due to the vessel's motion caused by waves, for instance.
The method of correcting measurement errors proposed in Japanese Patent Application Publication No. 1994-160522 however has a problem that it is difficult to determine the target strength TS of a single fish when the vessel is in motion (pitches, rolls or heaves) due to waves or wind, for instance, because this method does not make it possible to freely vary the directions of transmitting and receiving beams to make up for movements of the vessel. Also, while the aforementioned Japanese Patent Application Publication No. 2000-46946 discloses an arrangement in which the transducer includes a large number of vibrating elements divided into three groups to form transmitting and receiving beams having sharp directivity, the transmitting and receiving beams are oriented in different directions under conditions where the vessel is in motion due to waves or wind, for instance. The difference in the directions of the transmitting and receiving beams produces a more conspicuous influence when the transmitting and receiving beams have sharper directivity. For this reason, there arises a problem that measurement errors increase if the target strength TS is corrected by the angles of incidence of the incoming echo signals determined based on the time differences among the echo signals picked up by the receiving beams formed by the individual vibrating element groups.